KEYWORDS: Holograms, Glasses, Digital holography, Contrast transfer function, High power microwaves, Chalcogenide glass, CCD cameras, Thin films, 3D image reconstruction, Optical engineering
Fingerprint analysis is a popular identification technique due to the uniqueness of fingerprints and the convenience of recording them. The quality of a latent fingerprint on a surface can depend on various conditions, such as the time of the day, temperature, and the composition of sweat. We first developed latent fingerprints on transparent and blackened glass slides by depositing 1000-nm-thick columnar thin films (CTFs) of chalcogenide glass of nominal composition Ge28Sb12Se60. Then, we used transmission-/reflection-mode multiwavelength digital holography to construct the topograms of CTF-developed fingerprints on transparent/blackened glass slides. The two wavelengths chosen were 514.5 and 457.9 nm, yielding a synthetic wavelength of 4.1624 μm, which is sufficient to resolve pores of depths 1 to 2 μm. Thus, our method can be used to measure the level-3 details that are usually difficult to observe with most other techniques applied to latent fingerprints.
The exact solutions of the set of nonlinear coupled differential equations pertaining to Bragg and non-Bragg
orders for the case of interacting angular spectra in a photorefractive medium are obtained numerically. An iron
doped lithium niobate is used as a photorefractive material, and the incident wavelength is taken to be 514 nm.
During traditional two-beam coupling within a photorefractive material, many non-Bragg orders may also be
generated. For solving the set of coupled equations numerically, interactions between the spectra of two Bragg
and two non-Bragg orders are considered. The spatial evolution of the spectra of these diffracted orders hence their
spatial profiles are studied numerically for different incident profiles.
The analysis of fingerprints is important for biometric identification. Two-wavelength digital holographic
interferometry is used to study the topography of various types of fingerprints. This topography depends on
several conditions such as the temperature, time of the day, and the proportions of eccrine and sebaceous sweat.
With two-wavelength holographic interferometry, surface information can be measured with a better accuracy
compared to single-wavelength phase-retrieving techniques. Latent fingerprints on transparent glass, a
forensically relevant substrate are first developed by the deposition of 50–1000-nm-thick columnar thin films,
and then analyzed using the transmission-mode two-wavelength digital holographic technique. In this technique,
a tunable Argon-ion laser (457.9 nm to 514.5 nm) is used and holograms are recorded on a CCD camera
sequentially for several sets of two wavelengths. Then the phase is reconstructed for each wavelength, and the
phase difference which corresponds to the synthetic wavelength (4 μm to 48 μm) is calculated. Finally, the
topography is obtained by applying proper phase-unwrapping techniques to the phase difference. Interferometric
setups that utilize light reflected from the surface of interest have several disadvantages such as the effect of
multiple reflections as well as the effects of the tilt of the object and its shadow (for the Mach-Zehnder
configuration). To overcome these drawbacks, digital holograms of fingerprints in a transmission geometry are
used. An approximately in-line geometry employing a slightly tilted reference beam to facilitate separation of
various diffraction orders during holographic reconstruction is employed.
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